The present invention relates to improvements in a system for measuring the spectral distribution of a light-wave signal such as a semiconductor laser.
The spectrum of a semiconductor laser has heretofore been measured by spectrometry through use of a spectrometer and a Fabry-Perot interferometer. Now that the spectral width of the semiconductor laser has been improved, however, the resolution for measurement with such spectrometry is insufficient. One possible high-resolution measuring method is a heterodyne method which utilizes a beat signal of two laser lights. This method is defective in that the light frequency of a reference local oscillation laser must be close to the frequency of the semiconductor laser under test and must be extremely stabilized.
On the other hand, there has been proposed a self-delay heterodyne or homodyne method according to which a portion of the laser output light to be measured is delayed to equivalently produce two independent laser lights mutually equal in the statistic property of their frequency fluctuation and the spectrum of a beat signal of these two laser lights, that is, the delayed light and the undelayed light, is measured, whereby the spectral distribution of the semiconductor laser is measured (Kikuchi, Ohkoshi and Nakayama, "A Novel Method of High-Resolution Measurement of Spectrum of Semiconductor Lasers", Institute of Electronics and Communication Engineers of Japan, Technical Meeting Report on OptoQuantum Electronics, OQE 80-50, 1980).
As described above, according to the prior art, a high-resolution measurement of the spectral distribution of a semiconductor laser requires lengthening of the optical fiber delay line, resulting in the shortcoming that the measuring instrument inevitably becomes bulky.
Especially, with the recent improvement of the performance of the semiconductor laser, there has been a strong demand for a system which permits a high-resolution measurement of the spectral distribution with a small instrument, but no specific techniques therefor have been proposed so far.